The present invention relates to a corrosion inhibiting additive and, more particularly, a corrosion inhibiting additive which is as effective as hexavalent chromium compounds but which do not have the health hazards associated with hexavalent chromium compounds.
Corrosion inhibitive compounds have long been used on, for example, metallic surfaces to inhibit corrosion thereof. U.S. Pat. No. 2,387,528 describes alkali earth metal chromates containing trivalent as well as hexavalent chromium as additives for metal protective pigments. U.S. Pat. No. 2,430,589 describes protective pigments comprising calcium chromate associated with minor additions of ferric, manganic or chromic oxides. U.S. Pat. No. 2,902,394 describes the use of soluble chromium containing compounds used in aqueous metal treating or rinsing solutions applied to metal surfaces or to the conversion coating onto metal surfaces to improve corrosion resistance. U.S. Pat. No. 3,063,877 describes aqueous solutions for treating metal surfaces to, in part, improve corrosion resistance, which are prepared by partially reducing a dissolved hexavalent chromium compound with formaldehyde. U.S. Pat. No. 3,279,958 describes rinsing of phosphate, chromate and other chemical conversion coatings on metal surfaces with a dilute aqueous acid solution of a chromium chromate complex followed by a water rinse. The complex is prepared by treating aqueous chromic acid solution with an organic reducing agent to reduce a portion of the hexavalent chromium to the trivalent state.
In the aerospace industry, aluminum alloys achieve their high strength to weight ratio by inclusion of such additional elements as copper, silicon, chromium, manganese, zinc and magnesium. The presence of these elements in high strength aluminum alloys make them more susceptible to corrosion attack than pure aluminum. These high strength aluminum alloys are, therefore, generally protected in service by use of corrosion inhibitive compounds based on hexavalent chromium. These compounds include barium or strontium chromate particles used as inhibitive pigments and adhesives, paints and primers; chromic acid, which is used to produce a chromium rich conversion coating; and sodium and potassium dichromate, which are used as sealing compounds for anodized films.
All forms of hexavalent chromium are recognized by the United States National Institute of Environmental Health Sciences as a Group 1 known human carcinogen. Accordingly, the use of corrosion inhibiting compounds which contain forms of hexavalent chromium are subject to stringent regulation and control. It would be very beneficial to eliminate hexavalent chromium as a corrosion inhibiting additive to corrosion inhibitive compounds as described above.
U.S. Pat. No. 5,030,285 describes a substitute corrosion inhibiting pigment for hexavalent chromium compounds. The corrosion inhibiting additive includes a combination of ferric phosphate and ferrous phosphate. While the proposed corrosive inhibiting additive does not suffer from the possible health problems associated hexavalent chromium compounds, it has not proved to be particularly effective in inhibiting corrosion, particularly of metals, when used as an additive in inhibitive pigments in adhesives, paints and primers, or as a conversion coating.
U.S. Pat. No. 6,537,678 discloses a non-carcinogenic corrosion inhibiting additive comprising an anodic corrosion inhibitor and a cathodic corrosion inhibitor. The inhibiting additive provides protection against both localized and general corrosion. While the corrosion inhibiting additive is effective and does not suffer from the health problems associated with hexavelant chromium compounds, it has been found that the performance of anodic and cathodic corrosion inhibitor used in the inhibiting additive is limited by the solubility of the anodic and cathodic corrosion inhibitors.
Naturally, it would be highly desirable to provide corrosion inhibitors which can be used and substituted for hexavalent chromium inhibitors so as to avoid potential health hazards while at the same time provide effective corrosion protection on metal surfaces, particularly, high strength aluminum alloys used in aerospace applications.
Accordingly, it is the principle object of the present invention to provide a corrosion inhibiting additive which is non-carcinogenic.
It is a particular object of the present invention to provide a corrosion inhibiting additive as set forth above which is effective in preventing corrosion attack on metals.
It is a further object of the present invention to provide a corrosion inhibiting additive as set forth above which is particularly effective when applied to high strength aluminum alloys.
It is a still further object of the present invention to provide a corrosion inhibiting additive as set forth above which is effective against both general corrosion and pitting corrosion.
Further objects and advantages of the present invention will appear hereinbelow.